The present invention relates to a multilayer structure which functions as a photocatalyst and can render its surface hydrophilic upon irradiation with ultraviolet rays or visible rays. The present invention further relates to a process for producing the same.
Members comprising a substrate coated with a titanium oxide film functioning as a photocatalyst are used in various applications. Such members are applicable to various articles so as to take advantage of the hydrophilicity resulting from their photocatalytic function for various purposes such as, e.g., antifogging, cleaning by rain, cleaning by water washing, and antifouling. For example, in the case of articles wherein the substrate is a glass plate, such as, e.g. , windshields and sideview mirrors for vehicles, window glasses for buildings, and mirrors, a technique is used in which the substrate is coated with a photocatalyst film whose surface becomes hydrophilic and is thereby prevented from being clouded with steam or covered with adherent waterdrops.
JP-A-10-278165 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a technique in which a metal target comprising titanium as the main component is used to conduct sputtering in an oxygen-containing atmosphere and the resulting deposit is heated to thereby form on a glass plate a photocatalyst film having anatase crystals and excellent resistance to abrasion and wearing.
However, in order for the technique described above to form a titanium oxide film having photocatalytic activity, it is necessary that a titanium oxide film should be deposited on a substrate heated to 300xc2x0 C. or higher or that a titanium oxide film which has been deposited on a substrate having room temperature should be heated to a temperature as high as 600xc2x0 C. or higher. Furthermore, there has been a problem that in the reactive sputtering in which a titanium metal target is used in an oxygen-containing atmosphere, the glow discharge becomes unstable as an electrically insulating oxide accumulates on the titanium metal surface and, as a result, a titanium oxide film cannot be stably obtained.
JP-A-10-310653 discloses a technique in which an anatase titanium oxide film or rutile titanium oxide film is deposited on a glass plate by the sol coating method, organic titanate method, electron beam vapor deposition method, or the like. There is a description therein to the effect that the energy band gap between the conductive band and the valence band in a titanium oxide film functioning as a photocatalyst is regulated by incorporating a metal oxide selected from ruthenium oxide, cobalt oxide, cerium oxide, chromium oxide, rhodium oxide, and vanadium oxide to thereby effectively obtain hydrophilicity.
However, the technique described above has had a problem that a step of heating at a temperature as high as 350xc2x0 C. or higher is necessary for obtaining a crystalline titanium oxide film having photocatalytic activity, resulting in a complicated production process.
JP-A-1-92176 discloses a technique in which a titanium oxide photocatalyst film is deposited on a glass plate by reactive sputtering using titanium metal as a sputtering target and this titanium oxide film is doped with ions of a metal such as niobium, cobalt, or chromium by ion implantation to improve catalytic activity.
However, the technique described above has had drawbacks that an expensive ion injector is necessary for injecting metal ions in a glass by ion injection, and that it is virtually difficult to deposit a photocatalytically active film on a glass having a relatively large area, such as a window glass for buildings, etc., on an industrial scale.
The present invention has been achieved in order to overcome the problems described above.
One object of the present invention is to provide a multilayer structure comprising a substrate and formed thereon a film having satisfactory photocatalytic activity and practical antifouling properties.
Another object of the present invention is to provide a process for producing a multilayer structure comprising a substrate and formed thereon a film having photocatalytic activity and antifouling properties.
According to the first embodiment of the present invention provides, there is provided a process for producing a multilayer structure comprising a substrate and formed thereon a film mainly comprising titanium oxide and having photocatalytic activity, which comprises sputtering a conductive sinter target obtained by sintering a mixture of particles of titanium oxide and particles of at least one metal oxide selected from the group consisting of niobium oxide, tantalum oxide, vanadium oxide, zirconium oxide, tin oxide, chromium oxide and copper oxide, in an atmosphere capable of having a regulated vacuum to form the film mainly comprising titanium oxide and having photocatalytic activity on the substrate.
The film mainly comprising titanium oxide (hereinafter referred to as xe2x80x9ctitanium oxide filmxe2x80x9d) in the present invention is obtained by sputtering, in an atmosphere capable of having a regulated vacuum, a conductive target obtained by sintering a mixture of particles of titanium oxide as the main component and particles of at least one metal oxide, as a minor component, selected from the group consisting of niobium oxide, tantalum oxide, vanadium oxide, zirconium oxide, tin oxide, chromium oxide and copper oxide. The surrounding gas when sputtering that can be used is an inert gas such as argon or a mixed gas comprising an inert gas and oxygen.
Especially when an oxygen-containing atmosphere having a reduced pressure is used in order to sputter a titanium oxide film which does not have a considerable deficiency of oxygen as compared with the stoichiometric amount and is transparent in the visible region, the target surface is inhibited from being covered with an electrically insulating substance because a small oxygen amount is sufficient for the atmosphere.
The target used in the present invention, which is an oxide sinter containing titanium oxide as the main component, is electrically conductive in such a degree that electric current can flow through the surface thereof. It is preferred that a direct current glow discharge be used to bombard the conductive target and thereby deposit a film. This is because a direct current power source, which is relatively inexpensive, can be used to generate a stable glow discharge and deposit a photocatalytically active titanium oxide film on a substrate.
The target having conductivity for use in the present invention can be obtained by sufficiently mixing particles, preferably fine particles, of titanium oxide with particles, preferably fine particles, of at least one metal oxide selected from the group consisting of niobium oxide, tantalum oxide, vanadium oxide, tin oxide, chromium oxide and copper oxide, and forming the powder mixture into a given shape by pressure molding. Conventional cold pressing or hot pressing method can be used for this pressure molding.
The conductivity of the target for use in the present invention is preferably such that the surface resistance thereof is 500 xcexa9/xe2x96xa1 or lower, in order to maintain a glow discharge for sputtering the target over a long period of time to thereby form a titanium oxide film in a stable manner. If the surface resistance exceeds 500 xcexa9/xe2x96xa1, it is not preferable in that the glow discharge is apt to be interrupted. From the standpoint of even more stably maintaining a glow discharge over a long period of time, the surface resistance of the target is more preferably 50 xcexa9/xe2x96xa1 or lower.
In the present invention, the formation of a photocatalytically active titanium oxide film is preferably conducted at a substrate temperature of 170xc2x0 C. or higher. If the substrate temperature is lower than 170xc2x0 C., it is undesirable in that the titanium oxide film obtained has insufficient resistance to abrasion and wearing and has low photocatalytic activity. From this standpoint, the substrate temperature is more preferably 200xc2x0 C. or higher.
On the other hand, if the substrate temperature exceeds 270xc2x0 C. during film formation, the titanium oxide film tends to have a higher degree of anatase crystal orientation and, unexpectedly, the photocatalytic activity thereof decreases gradually. From this standpoint, the substrate temperature during film deposition is preferably 270xc2x0 C. or lower, more preferably 250xc2x0 C. or lower.
Preferred metal oxides to be contained in titanium oxide in the present invention are niobium oxide, vanadium oxide and tantalum oxide because these metal oxides are effective not only in relatively easily maintaining a glow discharge necessary for sputtering the sinter target but in obtaining a titanium oxide film having enhanced photocatalytic activity. Of those, niobium oxide is most preferable.
According to the second embodiment of the present invention, there is provided a multilayer structure comprising a substrate and formed thereon a film mainly comprising titanium oxide and having photocatalytic activity (hereinafter referred to as xe2x80x9ctitanium oxide filmxe2x80x9d), wherein the film contains as a minor component at least one metal oxide selected from the group consisting of niobium oxide, tantalum oxide, vanadium oxide, zirconium oxide, tin oxide, chromium oxide and copper oxide and is substantially amorphous when analyzed by X-ray diffraction method.
In the case where the titanium oxide film formed is composed of crystals, many grain boundaries are present therein. Electron-hole pairs which have been excited by light in the titanium oxide film are trapped by many lattice defects present on those grain boundaries, so that the number of electron-hole pairs present on the film surface, which contribute to photocatalytic activity, is reduced. Because of this, such a titanium oxide film has reduced photocatalytic activity. In contrast, the multilayer structure of the present invention is reduced in the trapping since it mainly comprises an amorphous matrix. This constitution is thought to bring about a high photoexcitation efficiency. The multilayer structure of the present invention therefore has high photocatalytic activity.
The titanium oxide film is characterized in that it is less apt to have a crystalline structure because the titanium oxide contains a small amount of one or more metal oxides.
In the present invention, the content of the metal oxide(s) as a minor component in the titanium oxide is preferably 1% by weight or higher, more preferably 2.5% by weight or higher. If the content of metal oxides as a minor component in the film is lower than 1% by weight, it is undesirable in that not only stable photocatalytic activity is difficult to obtain but also the target is less apt to retain conductivity (electricity-passing properties) in a degree so as to conduct sputtering in a stable manner, making it difficult to stably form a titanium oxide film.
On the other hand, the content of the metal oxide(s) as a minor component is preferably 10% by weight or lower, more preferably 5% by weight or lower. This is because if the content of the metal oxide(s) exceeds 10% by weight, the content of titanium oxide decreases in relative amount, resulting in lowering photocatalytic activity.
In the present invention, preferred metal oxides as the minor component are niobium oxide, vanadium oxide and tantalum oxide from the standpoints of obtaining high photocatalytic activity and forming a stable glow discharge to form a titanium oxide film with satisfactory reproducibility. Especially preferred from such standpoints is niobium oxide.
Usable substrates include ones made of inorganic materials such as glasses and ceramics and ones made of organic materials such as plastics because in the process of the present invention, film formation on substrates is possible at relatively low temperature. Examples of applications of such substrates include window glasses for buildings and vehicles, exterior and interior materials for buildings, Braun tubes, PDP displays, liquid-crystal display panels, optical recording media, magnetic recording media, and members or housings of domestic electrical appliances or OA apparatus.
In the case where a glass plate, especially one containing alkali components, such as a soda-lime silicate composition, is used as a substrate, it is preferred to dispose, between this glass substrate and a titanium oxide film, a primer film serving to prevent the alkali components of the substrate from dissolving in the titanium oxide film. From the standpoint of further improving antifouling properties, it is preferred to partly or wholly coat the titanium oxide film of the present invention with a hydrophilic film.